MINOR: stconn: Stop to set SE_FL_ERROR on sending path

MINOR: stconn: Stop to set SE_FL_ERROR on sending path

It is not the SC responsibility to report errors on the SE descriptor. It is
the endpoint responsibility. It must switch SE_FL_ERR_PENDING into
SE_FL_ERROR if the end of stream was detected. It can even be considered as
a bug if it is not done by he endpoint.

So now, on sending path, a BUG_ON() is added to abort if SE_FL_EOS and
SE_FL_ERR_PENDING flags are set but not SE_FL_ERROR. It is trully important
to handle this case in the endpoint to be able to properly shut the endpoint
down.